Phospholipids make a vital contribution to the characteristic permeability of cell. It has become apparent that quantitative, and even qualitative, changes of their content in cell membranes can influence the movement of molecules between the cell and its environment. Recently, we have found that a phospholipase enzyme can specifically increase the permeability of two 'model' epithelial membranes to sodium. An endogenous phospholipase has been isolated from frog skin and toad urinary bladder from which it is secreted and in the instance of frog skin, this process is increased by the hormone aldosterone. In the toad urinary bladder we have preliminary data which suggests a link between the production of phospholipase and the actions of aldosterone, as the effects of this hormone on Na transport are inhibited by mepacrine, an inhibitor of phospholipase A2. In preliminary experiments we have also identified a phospholipase in the mammalian kidney and we now wish to isolate and characterize it with respect to its activity in relation to pH, the presence of monovalent and divalent ions, its tissue storage, possible secretion and metabolism. The isolated enzyme will be tested to see if it can alter the permeability of our model epithelia. As such an enzyme can be contributing the mechanism of action of mineralocorticosteroids, especially aldosterone, the effect of such adrenal hormones on the phospholipase content of the kidney will be studied. Since phospholipase A2 is the rate limiting step in the production of prostaglandins, the effects of these hormones on prostaglandin production in relationship to the phospholipase will be studied both in the mammalian kidney and in the toad urinary bladder. These studies may provide important information about the mechanisms involved in hormonal control or renal fluid and electrolytes and may have practical implications in the diagnosis and treatment of diseases that affect the kidney and the design of drugs that modify its function.